The present invention relates to a fuse link system for Transient Voltage Surge Suppression (TVSS) devices, namely one which utilizes a fuse link system to provide the effective removal of electrical potential across a circuit component after a component failure.
Transient Voltage Surge Suppression (TVSS) systems are placed between a power line or source of power, and electrical equipment receiving power from the source of power, to protect the equipment in the event of a transient over-voltage condition. TVSS systems typically utilize Metal Oxide Varistors (MOV""s) or Silicon Avalanche Diodes (SAD""s) between an Alternating Current (AC) power or phase line and a neutral or ground line, to suppress a voltage transient. A plurality of MOV""s are typically mounted on a printed circuit board and the MOV""s become conductive when a transient voltage is experienced. During the temporary over-voltage condition, the MOV, in its conductive state, discharges the potentially damaging transient to the neutral or ground line.
There will typically be a column and row configuration wherein one or more MOV""s for example are wired in series, are electrically connected in parallel with a second plurality of MOV""s wired in series. When a transient voltage is experienced, the combination of MOV""s may participate in the discharge of the transient voltage to the neutral or ground line.
Fuses are typically electrically connected in series with the MOV""s to provide a cutout or fuse mechanism in the event that an MOV""s limits are exceeded and the MOV fails, and in order to prevent significant damage to all of the MOV""s when only some are affected. Such fuses are designed or intended to prevent heating of the surge suppression device and the printed circuit board, and ignition thereof. Typical off-the-shelf fuses will not work adequately with heavy duty surge suppression circuits because of the high transient currents that surge components are capable of diverting. A typical twenty millimeter (20 mm) diameter MOV is capable for instance of diverting approximately twelve thousand (12,000) amperes during an industry (Institute of Electrical and Electronics Engineers xe2x80x9cIEEExe2x80x9d) standard 8/20 microsecond pulse.
In high energy operating environments, the limits of MOV""s may be exceeded or far exceeded, causing some MOV""s to fail short and burn. Since the failure mode for typical TVSS surge suppression components is low impedance and they are typically placed across the line (from phase or power to neutral or ground), high fault currents and arcing will result.
If the TVSS system is designed with fuses to take full advantage of the current capabilities of the surge suppression devices, the fuses will have to survive the same large current transient pulse that the surge suppression devices are designed to survive. A fuse sized to accomplish this task will have a high steady state current rating that may not open in time during a fault condition to keep upstream breakers from tripping.
If the fuses are sized to keep any upstream breakers from tripping they will open during transient conditions which have less magnitude than what the suppression devices are designed to withstand. This type of fuse coordination will take surge suppression devices off line that are not damaged therefore reducing the overall capabilities of the system. Since the fuse as well as the surge suppression device will be the limiting factor in the overall energy rating of the device many fuses will be required and therefore the size and cost of the fuses will be prohibitive.
The high fault currents or arcing cause high temperatures, which in turn cause most material in the vicinity of the fault (including the MOV""s) to combust.
A high transient current therefore creates unusual environmental conditions and concerns that typical electrical fuses are not sufficient to handle. Further contributing to the difficult operating and performance environment for higher energy TVSS systems is that such TVSS systems are typically placed or contained in a housing, so that products of combustion are contained or substantially contained within the housing. The needs of customers is driving the size requirements of the TVSS system to smaller and smaller sizes, which further contributes to the problem.
While circuit boards provide insulating functions during many applications and conditions, they do not reliably provide sufficient and 5 reliable insulating capabilities in the unique situation presented by high energy TVSS systems during a failure condition, at least in the proximity near the fuses and/or near the surge suppression devices or components. Circuit boards which inherently have a potential from one section of the circuit board to another, provide a surface or platform for the products of combustion to create a conductive path leading to unwanted additional damage to the TVSS system.
A fuse system is required to allow the components to survive a significant transient current, typically at lease 8,000 ampere (in the 8/20 industry standard test), but which will clear or open in the even of a is fault without tripping any feeding breakers or causing a fire/smoke/heat hazard. Since each system is typically comprised of multiple high current components, there will preferably be multiple fuses.
There have been prior fusing methods attempted, such as small traces which feed suppression elements. However, the products of combustion deposited on the printed circuit board during the clearing of the fuse trace may create a path on the printed circuit board through which current will continue to travel due to the close proximity of the fuse traces to or on the surface.
Embodiments of this high voltage transient voltage surge suppression fuse link system are therefore intended to provide an improved surge suppression fuse system which greatly reduces or eliminates some of the forenamed problems in fusing systems which provide fusing across a potential which is all located on a single circuit.
There is also a need to provide an alarm to the operator of said systems if the system becomes partially inoperable or if some or all of the surge suppression devices have become inoperable or destroyed.
When a fault condition is encountered in a TVSS system, after one or more of the fuses or fuse links opens, it is desirable that the surge suppression system signal to the user or operator that a reduced suppression condition is present, so that corrective action may be taken or planned. The signalling of a condition is typically accomplished by a switch which is either energized or de-energized when the one or more fuse links is lost. The switch output is typically a Form C type of switch which will change state when the alarm system is energized or de-energized.
Any one of a number of known remote monitoring systems may be used to access the Form C switch via a three pole terminal block located on the surge suppression circuit.
The surge suppression industry is moving toward smaller and smaller surge suppression systems and the amount of space required by each component, including the switch, is becoming more and more critical.
It is also a design criteria or goal that said switch minimize the heat contributed to the surge suppression system during normal operations. For this and other reasons, a lower power miniature relay is usually the design choice for surge suppression systems, because larger relays result in unnecessary heat.
Low power miniature relays typically employ a small permanent magnet internal to the relay, the use of which creates issues and problems since the relays are expected to operate in magnetic fields of varying intensity, including some very strong magnetic fields. For example, a larger magnitude transient may be ten thousand (10,000) amperes or greater. A strong magnetic field may weaken the internal magnet in the relay and under certain conditions, may render it inoperable.
The elimination of the magnets from the alarm system makes the alarm and signaling system more reliable and eliminates the effect of magnetic fields on the system.